There is a reference at ORC stating he owned the works (and only one built) Chevron B4 from 1968.
https://www.oldracingcars.com/chevron/r
(I read elsewhere that the car has recently changed owners and has been fully restored. Maybe the current owner has not info. (current owner mentioned in this link)
https://au.news.yahoo.com/historic-m...173007477.html
There is also a full list of results for Roy detailing his European racing from 1966 to 1975 in a variety of cars here:
https://www.racingsportscars.com/dri...ohnson-GB.html